The present invention is related to a data recording and readingout system which records data on an optical recording medium, and readout data from the optical medium. More precisely speaking, the present invention is directed to such a data recording and readingout system capable of largely increasing a recording capacity of the optical recording medium.
Conventionally, optical recording media typically known as CDs and DVDs have been widely used as recording media employed so as to record thereon digital data. Very recently, such optical recording media having larger storage capacities and higher data transfer rates have been actively developed.
In the above-described optical recording media, wavelengths “λ” of laser beams which are employed so as to record and readout data are shortened and numerical apertures “NA” of objective lenses are increased so as to narrow diameters of laser beam spots, so that recording capacities of the optical recording media may be increased (refer to, for instance, Japanese Patent Publication JP-A-8-235638).
In an optical recording medium, if lengths of recorded marks recorded on the optical recording medium and a length between the adjoining recorded marks, namely, a length of a region where a recorded mark is not formed (will be referred to as “blank region” hereinafter) becomes smaller than a limit of resolution, then data cannot be readout from this optical recording medium.
A beam spot diameter of a laser beam is determined based upon a wavelength “λ” of the laser beam and a numerical aperture “NA” of an objective lens used to converge the laser beam, and thus, becomes “λ/NA.” At this time, in such a case that a repetition frequency between a recorded mark and a blank region, namely, a spatial frequency becomes larger than, or equal to λ/2NA, both the recorded mark and data recorded on the blank region cannot be readout.
As a consequence, a length of a recorded mark and a length of a blank region which correspond to a readable spatial frequency become larger than, or equal to “λ/4NA”, respectively, and when the laser beam having the wavelength “λ” is collected onto a surface of an optical recording medium by the objective lens having the numerical aperture “NA”, both the recorded mark having the length of “λ/4NA” and the blank region having the length of “λ/4NA” may constitute both the shortest recorded mark and the shortest blank region, which can be read.
As previously explained, in the case that data is readout, there is a limit of resolution capable of readingout data, and also, there are certain limitations in a length of a recorded mark and a length of a blank region under which the data can be readout.
Accordingly, even when both small recorded marks and small blank regions are formed and data could be recorded, the recorded data cannot be readout. As a result, both the lengths of the recorded marks and the lengths of the blank regions which can be formed when the data are recorded on the optical recording media are necessarily limited, so that the recording capacities of the optical recording media can be very hardly increased.